PTGS2 and gastric cancer: The temporal trend of COX-2 methylation levels during GC development was explored by dividing the pre- and post-GC samples into 5 groups (5–10 years pre-GC, 1–4 years pre-GC, GC diagnosis year, 1–4 years post-GC and 5–10 years post-GC) according to the time interval between sample collection and GC diagnosis.